1. Field of the Invention
The present invention relates to an electric double-layer capacitor.
2. Description of the Related Art
Generally, a boundary across which two different phases of a solid and a liquid contact each other is associated with an electric double layer which comprises positive and negative electric charges that confront each other across a very small distance therebetween. When an electric field is applied between a pair of polarized electrodes, each made of a solid, which confront each other across a separator that is impregnated with an electrolytic solution, the assembly can be used as an electric double-layer capacitor.
One conventional electric double-layer capacitor has a plurality of electrode elements comprising respective current collectors and polarized electrodes disposed on surfaces of the current collectors, the electrode elements being stacked together with a separator interposed therebetween, forming a stacked assembly. Current collecting leads extending respectively from the current collectors are put together into lead groups of respective polarities, which are connected to respective electrode terminals.
Specifically, the current collecting leads extending respectively from the current collectors are each in the form of a strip of aluminum foil. The current collecting lead of each of the current collectors is displaced toward one side of the face of the polarized electrode which confronts the corresponding electrode terminal. The electrode elements are alternately reversed and stacked together with a separator interposed therebetween, forming a stacked assembly. Adjacent two of the electrode elements have their current collecting leads positioned respectively on different sides thereof. The current collecting leads positioned on the same sides are put together into stacked lead groups of respective polarities, which are connected to the respective electrode terminals.
It has heretofore been known to connect the current collecting leads and the electrode terminals by an ultrasonic welding process. According to the ultrasonic welding process, each stacked electrode group is held against a flat surface of the corresponding electrode terminal, and a pressing member which applies ultrasonic vibrations is pressed against the stacked electrode group in a direction across the current collecting leads to generate fusing heat at the joined surfaces of the current collecting leads and the electrode terminal for thereby joining the current collecting leads altogether to the electrode terminal by way of solid phase diffusion.
If the capacitance of the electric double-layer capacitor is to be increased by increasing the number of polarized electrodes, then the number of current collecting leads is necessarily increased. When the current collecting leads are to be connected altogether to the electrode terminals by ultrasonic welding, however, the ultrasonic energy may not sufficiently be transmitted because of the increased number of current collecting leads. As a result, some of the current collecting leads, particularly those positioned closely to the electrode terminals, i.e., remotely from the pressing member for applying ultrasonic vibrations, tend to be separated, rather than being firmly joined, resulting in an incomplete electric connection between the current collecting leads and the electrode terminals.
Attempts have been made to increase the output power (electric power) of the ultrasonic energy applied in ultrasonic welding as the number of current collecting leads is increased. However, since the current collecting leads and the electrode terminals are made of aluminum, the increased output power tends to produce more oxide film on the adjacent boundary surfaces of the current collecting leads and the electrode terminals. As a consequence, the resistance of the portions of the current collecting leads which are joined to the electrode terminals tend to increase and regions around those portions of the current collecting leads which are joined to the electrode terminals are liable to crack.
The electric double-layer capacitor of the type described above is housed in an outer case and sealed by a lid. The electrode terminals are highly hermetically mounted on the lid before the current collecting leads are connected to the electrode terminals. When the current collecting leads are connected altogether to the electrode terminals by ultrasonic welding with increased output power thereof, the applied ultrasonic vibrations are detrimental to the hermetically sealed connection between the lid and the electric terminals.